CN101571494A - Crack detection system - Google Patents

Abstract

A crack detection system for detecting cracks in a load engineered structure (1) is provided. The crack detection system comprises a light source (17), an optical fiber (6) introduced into the structure (1), and means (19) for coupling light from the light source (17) into the optical fiber (6). The optical fiber (6) has a diameter smaller than 75 μm.

Description

Crack Detection System

The present invention relates to a crack detection system for detecting cracks in a loaded engineering structure. Load engineering structures such as wind turbine rotor blades, aircraft wings, propellers, helicopter rotors, vehicle structural components, concrete buildings, concrete dams, etc.

Cracks and failures in such load-engineered structures can have serious consequences. Early warning of propagating fractures saves lives and property.

WO2006/012927A1 discloses a method and a device which can detect cracks in a wind turbine rotor blade. The device for monitoring the state of a rotor blade on a wind power installation measures structure-borne noise by means of at least one displacement sensor arranged on the rotor blade. The spectrum is determined from the sensor signal and compared to a reference spectrum corresponding to the defined state of the lesion and other specific states. By comparing the determined frequency spectrum with a reference frequency spectrum, the state of the rotor blade can be determined.

Another method for detecting cracks in wind turbine rotor blades was described by Niels Preben Immerkaer, Ivan Mortesen, LM Glasfiber A/SLunderskov, November 2004. These authors describe providing three strands of optical fiber running parallel to the trailing edge of a wind turbine rotor blade. The distances of these fibers from the trailing edge were 2 cm, 4 cm and 6 cm, respectively. Cracks propagating from the trailing edge will damage fibers starting with the outermost fiber (ie, the fiber closest to the trailing edge). From the number of cracked optical fibers of the crack detection system, an emergency status regarding cracks can be determined. The fiber used is part of a fiber Bragg grating system (FBG). Such systems use a single optical fiber with a diameter of approximately 120 μm.

From the point of view of being superior to the state of the art, it is a first object of the present invention to provide a crack detection system which facilitates the detection of cracks in load engineering structures, especially for crack detection in wind turbine rotor blades.

A second object of the present invention is to provide a method that facilitates the detection of cracks in engineering structures.

The first object is achieved by a crack detection system for detecting cracks in load engineering structures as claimed in claim 1 and the second object is achieved by a method for detecting cracks as claimed in claim 12 . The dependent claims contain further developments of the invention.

An inventive crack detection system for detecting cracks in a load engineered structure includes a light source, an optical fiber introduced into the structure, and a mechanism for coupling light from the light source into the optical fiber. The diameter of the optical fiber used in the inventive crack detection system is less than 75 μm.

As mentioned above, the diameter of the optical fiber used in the fiber grating system is about 120 μm. Furthermore, these fibers are extremely fragile. Fibers with diameters smaller than 75 μm are less brittle than these fibers, and their dimensions are similar to those found in typical fiberglass laminates. Fibers with diameters smaller than 75 μm are therefore stronger than those used in fiber grating systems and are especially useful in fiber-reinforced laminate structures, such as the casings of wind turbine rotor blades.

To increase the ability of these fibers to transmit light, they can be introduced into the structure in the form of fiber bundles. Such bundles can consist of up to a few hundred single fibers, which can make the system more robust, since the larger number of fibers gives the system redundancy and thus makes it less susceptible to damage.

Another advantage of bundled fibers is that if a crack occurs, all the fibers in a bundle will not break at the same time. As more and more fibers in the bundle break, the propagation of light through the bundle decreases. This is a simple and effective method for detecting degradation of engineering structures, especially useful if the degradation is slow to develop. However, even if the crack propagates rapidly, determining the fracture sequence of the fibers in the fiber bundle can provide an indication of crack propagation, which can be very useful for crack assessment.

With the use of single fibers with a diameter of less than 75 μm, for example around 50 μm in diameter, the fiber bundle is compatible with the surrounding fibers if the engineered structure is made of a fiber reinforced laminate. In addition, bundling the fibers into a bundle increases the ability to transmit light, which allows cracks to be detected with the naked eye by light emerging from a broken fiber bundle, especially at night. Thus, due to the cracks in the above-mentioned structure, it becomes possible to visually detect the cracks during the inspection of engineering structures by visually detecting the light rays escaping from the broken optical fiber bundle.

To ensure proper wetting by the adhesive or matrix material in laminated structures, the fiber optic bundles can be encased in a permeable hose, such as a braided hose of plastic fibers (usually thermoplastic polyester) or glass fibers. Alternatively, one or more strands may be wrapped around the bundle of fibers to form the enclosure. The single or multi-strand wires wrapped around the fiber bundle, or the permeable hose, can be heavily colored to make the fiber bundle easy to locate. This method prevents damage to the fiber optics if repair or maintenance work is later performed on the composite laminate structure.

A bundle of optical fibers, optionally enclosed in a permeable tube or surrounded by one or more strands, can be embedded in the laminate when fabricating the laminate, or by cutting a longitudinal groove into the surface and subsequently bonding the bundle of fibers Into this slot, it can be embedded eg in existing structures like wind turbine rotor blades. Alternatively, the fiber optic bundles can be glued to the outside or inside of the structure without slotting. If the fiber optic bundles are glued to the outside of the above structure, they will not be covered by paint, simplifying the visual detection of escaping light.

If more than visual detection is possible, an optical fiber or fiber bundle can be connected to the light detector. In addition, there may be a modulation unit connected to the first light source for modulating its light and a time gate unit connected to the detector for receiving the detector signal. In this case, the signal from the detector can be time-gated to suppress noise and any other interfering signals, thereby enhancing sensitivity. Furthermore, the time gates described above can be delayed relative to the modulated light from the light source, and the time delay can be varied. The time delay to produce the maximum register signal can be used to estimate the distance of the light source from the cut in the fiber or fiber bundle, which corresponds to the distance of the slit from the light source.

Alternatively or in addition, light detectors may be located at the ends of the optical fiber or fiber bundle opposite the ends where light is coupled into the fiber or fiber bundle. Then, a transmission coefficient determination unit that determines and monitors the transmission coefficients of the optical fibers or fiber bundles, in particular their respective transmission coefficients, can be used to detect a sudden decrease in the transmission coefficient, which indicates that a crack has opened in the blade.

If cracks are located eg in a wind turbine rotor blade such that the cracks open and close due to gravity as the rotor rotates, the determined transmission coefficient will vary periodically at a frequency corresponding to the rotor rotation frequency. Thus, the Fourier coefficients of the transmission coefficient time series corresponding to this frequency indicate the severity of the damage. Alternatively, relative differences in time series between 10% and 90% amounts can be monitored.

In order to allow the implementation of the mentioned method, a frequency detection unit may be present in a crack detection system which is connected to the transmission coefficient determination unit and designed to determine the frequency components in the transmission signal, said transmission coefficient determination unit using for receiving a transmission coefficient signal indicative of the transmission coefficient of an optical fiber or fiber bundle.

In yet another alternative, the light source may be a pulsed light that emits light pulses with a pulse length in the range of less than 500 ns. Furthermore, there may be a time delay determination unit which determines the time delay between the sending of a pulse by the light source and the reception of the corresponding pulse by the detector. The intensity of the light diffused from the cut in the fiber back to the light source, indicating structural cracks, can then be detected. A sudden increase in backscatter intensity can be taken as an indication that a crack has opened. The time delay from the emission of the pulse from the light source to the detection of the reflected light can be used to determine the distance of the cut in the fiber from the light source, and thus the location of the crack in the blade. A short pulse will determine that the reflected pulse can be separated from the original pulse.

The light source used in the inventive crack detection system may be any one of a light emitting diode (LED), a laser diode or any other suitable light source. The photodetectors can be photoresistors, photodiodes, or any other suitable photodetectors. Glass fibers or plastic fibers such as PMMA (polymethacrylate) fibers can be used as optical fibers.

If a crack, i.e. a cut in the fiber or fiber bundle, is detected by visual inspection, the light source can be switched off during the inspection to ensure a longer lifetime of the light source. On the other hand, if continuous monitoring is practiced, the light source can be switched off intermittently to prolong the life of the light source.

The inventive crack detection system can be used to implement the inventive method of detecting cracks in engineered structures. In this method, the transmission and reflection of light entering an optical fiber or fiber bundle is monitored and cracks are detected by sudden changes in transmission or reflection. In particular, reflections can be monitored if photodetectors are located at the same end of the fiber or fiber bundle, and the light source(s) are also located at that end. On the other hand, if a number of photodetectors are located at the far end of the fiber or fiber bundle relative to the end where the light source is located, the transmission can be monitored. Sudden changes in transmission or reflection can be reliable indicators of cracks in engineered structures.

When light is injected into an optical fiber or fiber bundle in the form of optical pulses, the time delay of the reflected optical pulse relative to the emitted optical pulse can be determined, thereby estimating the distance from the crack to the light source.

If the time series of the transmission coefficient or reflection coefficient is established, and the Fourier analysis of the time series is performed, the severity of the crack can be estimated. This is particularly advantageous if the engineering structure is a wind turbine rotor blade. If the cracks are positioned in the rotor blade such that they open and close due to gravity as the rotor rotates, the transmission or reflection coefficient will vary periodically with a frequency corresponding to the frequency of rotation of the rotor. Thus, the Fourier coefficient corresponding to this frequency indicates the severity of the damage. The higher the Fourier coefficient, the greater the estimated damage.

Alternatively, instead of performing Fourier analysis of the time series, the difference between the 10% quantile and the 90% quantile can be monitored. In other words, the entire range of transmission or reflection coefficient values is divided into 10 equal intervals, where the quantile points represent the boundaries between these intervals. If there is a large difference in the number of data points below the 10% quantile on the one hand and below the 90% quantile on the other hand, this implies a narrower distribution. However, a narrow distribution range implies a relatively constant transmission or reflection coefficient. If the cracks open and close due to gravity, the distribution of transmission or reflection coefficient values will expand, reducing the gap between the number of data points below the 10% quantile and below the 90% quantile. If the reduction is large, the crack is estimated to be more severe.

Other features, properties and advantages of the present invention will be apparent through the following description of the embodiments of the present invention in conjunction with the accompanying drawings.

Figure 1 shows the inventive crack detection system in the context of a wind turbine rotor blade.

Figure 2 shows a first alternative for arranging the optical fibers or fiber bundles of the crack detection system.

Figure 3 shows a second alternative for arranging the optical fiber or fibers of the crack detection system.

Figure 4 shows a third alternative for arranging the optical fibers or fiber bundles of the crack detection system.

Figure 5 shows a bundle of optical fibers encapsulated by a hose.

Figure 6 shows a fiber optic bundle encapsulated by a wire bundle that is wrapped around the fiber optic bundle.

FIG. 7 shows details of the crack detection system shown in FIG. 1 .

Figure 8 shows a second embodiment of the inventive crack detection system in the context of a wind turbine rotor blade.

Figure 9 shows details of a second embodiment of the crack detection system.

Figure 1 shows a first embodiment of the inventive crack detection system in the context of a wind turbine rotor blade. Note that a wind turbine rotor blade is only an example of an engineering structure in which a crack detection system may be used.

The wind turbine rotor blade 1 shown in FIG. 1 comprises a root 3 , a shoulder 5 adjoining the root in the outward direction of the blade, and an airfoil 7 extending from the shoulder to the blade tip 9 .

Furthermore, the blade 1 comprises a leading edge 11 and a trailing edge 13 .

The blade shown in Figure 1 is equipped with an inventive crack detection system. The crack detection system includes a plurality of optical fiber bundles 15, a light source 17 and a mechanism 19. The diameter of a single optical fiber in the plurality of optical fiber bundles 15 is less than 75 μm. The light source 17 is located in the root of the blade 1 in this embodiment, and the mechanism 19 is used to use the light source The light is coupled into the fiber bundle 15. The mechanism for coupling light into the fiber optic bundle 15 is indicated as block 19 in the figure. Suitable mechanisms for coupling light into an optical fiber or fiber bundle are known to those skilled in the art and are therefore not described here. The light source may be at least one incandescent lamp, one or more bright light emitting diodes, or at least one laser or laser diode.

Although fiber bundles 15, which may contain up to several hundred individual optical fibers, are used in this embodiment, the use of fiber bundles is not mandatory. Alternatively, single fibers with a diameter of less than 75 μm can in principle also be used. Whether to use a fiber bundle or a single fiber extending through the blade is determined by the amount of light that will be transmitted through the fiber bundle or the fiber, respectively.

The inventive crack detection system of the first embodiment also includes several photodetectors 21 for detecting light reflected back to the light source, in this embodiment one photodetector 21 for each fiber optic bundle 15 . However, it is also conceivable to use the light detector 21 for a subset of the fiber bundles 15, ie to connect more than one fiber bundle to one light detector.

Although the light source 17, the mechanism 19 for coupling light into the fiber optic bundle and the light detector 21 are shown at the root in Figure 1, they could also be located at a different location, such as the rotor hub to which the rotor blades are attached. This will provide the benefit that a single arrangement of light source, mechanism for coupling light into the fiber optic bundle and light detector is sufficient for all rotor blades of the rotor.

2 to 4 show three different options for arranging fiber bundles 15 or single fibers into the blade 1 . The figure shows a cross section of the blade 1 from the leading edge 11 to the trailing edge 13 .

In FIG. 2 the fiber optic bundle 15 is located outside the blade housing 23 . These fiber bundles can be fixed to the housing 23 by gluing them to the outer surface of the housing. In order to minimize the aerodynamic influence of the fiber bundle 15 on the blade 1, the diameter of the fiber bundle should be as small as possible, that is to say the fiber bundle should only consist of a few single fibers. Bonding on the outside of the blade shell 23 is particularly suitable if a single fiber is used instead of the fiber bundle 15 because its diameter of less than 75 μm prevents the single fiber from seriously affecting the aerodynamic properties of the blade 1 .

Another option for arranging fiber bundles 15 or single fibers is shown in FIG. 3 . As in Fig. 2, the fiber optic bundle 15 is glued to the casing 23 of the wind turbine rotor blade. However, unlike the option shown in FIG. 2 , the fiber bundles of FIG. 3 are glued to the inner surface of the casing 23 so that they do not have any influence on the aerodynamic properties of the blade 1 . The other aspects described respectively in relation to the fiber bundle 15 or fiber shown in FIG. 2 are equally valid for the fiber bundle or fiber shown in FIG. 3 .

Figure 4 shows a third option for arranging fiber bundles 15 or single fibers. In this arrangement, the fiber optic bundle 15 is integrated into the casing 23 of the rotor blade 1, which is typically a fiber reinforced laminate structure. This means that the optical fibers or fiber bundles 15 can be easily integrated into the housing when forming the laminated structure of the housing. The single fibers are smaller than 75 μm, especially in the range of about 50 μm, making these single fibers compatible with the fibers around typical fiberglass laminates used to manufacture wind turbine rotor blade casings.

In order to ensure proper wetting of the fiber bundles or optical fibers by the glue or liquid polymer used in forming the housing 23, the fiber bundles may be encased in a permeable hose 25, such as plastic fibers (usually thermoplastic polyester) or Fiberglass has a braided hose. FIG. 5 shows a cross-sectional illustration of a fiber bundle 15 consisting of a plurality of optical fibers 6 surrounded by a hose 25 . Alternatively, the fibers 6 of the fiber bundle 15 may be wrapped with one or more strands wrapped around the fibers or fiber bundle, as shown in FIG. 6 . If the housing 23 needs to be corrected or repaired, the above-mentioned hoses or wire harnesses can be colored in a thick color so that the optical fibers or optical fiber bundles can be easily positioned to prevent damage to the optical fibers or optical fiber bundles.

A method of detecting cracks using the crack detection system shown in FIG. 1 will now be described with reference to FIG. 7 . Figure 7 shows the light source 17, fiber bundle 15 and light detector 21 in more detail. To make the diagram simpler, the mechanism for coupling light into the optical fiber 15 has been omitted. The crack detection system comprises a modulation unit in the form of a pulse generator 29 for generating light pulses to be coupled into the optical fiber 15 . The light pulse generator 29 can act on the light after the light source 17 has emitted it. The above-mentioned pulse generator can be realized, for example, in the form of a shredding device like a louver, a cutting carousel or the like. Alternatively, the pulse generator 29 itself can act on the light source 17 so as to operate the light source 17 in a pulsed mode, ie cause the light source 17 itself to emit light in pulses of light. In the embodiment shown in Figure 7, a pulse generator 29 is connected to the light source 17 to provide control signals for operating the light source 17 in a pulsed mode with a pulse length in the range below 500 ns.

The pulse generator 29 is connected to a window generator 31 in a time gating unit 32 connected to the light detector 21 for receiving a signal representative of the detected light. If the signal reaches the time window defined by the window generator 31 , the time gate 32 passes the signal to the analyzer 33 . Otherwise, the signal from the photodetector 21 would not pass to the analyzer 33 . The window generator 31 is adjustable so that the length of the time window can be adjusted and the time difference between the transmitted light pulse and the center of the time window can be changed. Therefore, the time gate unit 32 is used as a delay determination unit.

During use of the crack detection system, the time window 31 is initially large enough to pass every signal from the photodetector to the analyzer. If any cracks appear at the end of the blade, the light pulses from the light source 17 will pass through the fiber optic bundles 15 and leave them at the tip 35 of the fiber bundle, so that the light detector 21 no longer detects any light. However, if a slit cuts the fiber bundle or a portion of the fiber bundle, this will cause reflections such that light is reflected from the slit back to the at least one photodetector 21 . To achieve this, the fiber bundles are placed close enough to each other that in any case a crack exceeding a given critical size will sever the fiber bundle.

As soon as the analyzer 33 detects the reflected light, the time window is reduced such that the length of the time window is sufficiently smaller than the time difference between two light pulses. Then, the offset of the center of the window relative to the time at which the light pulse was emitted is varied and the intensity received by the light detector 21 is monitored. The distance of the notch in the fiber bundle 15 from the detector 21 can be estimated from the time difference between the transmitted light pulse and the center of the time window when the detected light exhibits a maximum value. Thus, the inventive crack detection system not only indicates the presence of cracks, but also hints at the location of cracks in the blade. However, the time gate 32 and the pulse generator 29 can be omitted if it is only necessary to detect the presence of cracks, in which case pulsed emission of light is unnecessary.

Fiber bundles 15 with up to several hundred individual fibers 6 (eg 400 fibers) may be used to deliver large amounts of light. If a crack occurs, a small portion of this light will be reflected to the photodetector. The remaining light will leave the fiber optic bundle and the rotor blade 1 via the cut and the slit, respectively. In this case, the user can locate the crack by optical inspection with the naked eye, especially at night. At this point, the light detector is only used to trigger the alarm that starts the above-mentioned check. Furthermore, if inspections are performed on a regular basis, due to the use of large fiber optic bundles, crack detection can be based entirely on visual inspection at night with the naked eye, thus eliminating the need for photodetectors 21 entirely.

On the other hand, if a crack detection system such as that shown in Figure 7 is used, it may not be necessary to use a fiber bundle of several hundred fibers, if the sensitivity of the photodetector is large enough to detect a small number of fibers or a single fiber with a diameter of less than 75 μm. reflected light, the use of fiber optic bundles is not even required at all. Then, depending on whether automatic crack detection or crack detection by conventional visual inspection is desired, it is determined whether to keep a simple crack detection system with a larger fiber bundle or a more complex crack detection system with a smaller fiber bundle or a single fiber.

However, not only the large amount of light transmitted via fiber bundles with up to several hundred fiber bundles is an advantage of fiber bundles, but also the fibers are constantly damaged one by one, in other words constantly in number in the area where the fiber bundles are located. Increased damage is also an advantage of fiber optic bundles. When more and more fibers are broken, an increase in reflected light intensity can be detected at the source location, or a decrease in transmission can be detected if the photodetector is located at the far end of the fiber bundle relative to the source. This is a simple and effective way to measure degradation. The signal is modulated (sinusoidally) as each ruptured fiber optic crack opens and closes during rotor rotation. However, a single fiber only allows detection of binary signals, such as reflection off-reflection on (or transmission off-transmission on).

Figures 8 and 9 show a second embodiment of the inventive crack detection system. The difference between the second embodiment and the first embodiment lies in the position of the photodetector 21 . Unlike the first embodiment, the light detector 21 is located at the distal end of the fiber optic bundle 15 instead of the light source end. This means that if a crack cuts the fiber optic bundle 15, the light detector 21 will not detect an increase in light but a decrease in light due to the decrease in transmission due to the crack. The above-mentioned crack detection system may be equipped with a transmission coefficient determination unit 37 (see FIG. 9 ) which transmits the light intensity via the optical fiber bundle 15 on the basis of the light intensity detected by the respective photodetector 21 and the light intensity known from the light source 17. to determine the transmission coefficient. A time series generator 38 is connected to the transmission coefficient determination unit 37 for receiving the transmission coefficients and forming a time series of the transmission coefficients. Furthermore, the frequency detection unit 39 is connected to the time series generator 38 for receiving the generated time series. The frequency detection unit 39 performs Fourier analysis of the time series, and obtains the Fourier coefficients of the time series corresponding to the rotational frequency of the rotor. This transmission coefficient is then output to the analysis unit 41 which is connected to the frequency detection unit 39 and performs an estimation of the severity of the crack based on the Fourier coefficients. Since the transmission coefficient of each fiber bundle varies from a maximum value (when the crack is closed) to a minimum value (when the crack is open) with a periodicity corresponding to the rotor rotation frequency, if the crack acts on the This is possible if the blades are opened and closed by the force of gravity on the blades.

A large Fourier coefficient refers to the large influence of the fracture on the transmission coefficient, which allows the fracture to be estimated as severe.

A possible alternative to the Fourier analysis of the time series is to monitor the difference between the 10% and 90% quantiles of the transmission coefficient values of the time series. The 10% quantile represents all values that occur less than 10% of the time. The 90% quantile, on the other hand, represents all values that occur with 90% probability. If the ratio of the 10% quantile to the 90% quantile changes, it means there is a crack. The degree of change indicates the severity of the crack. The reason for this is that due to the already mentioned force of gravity, the number of high transfer coefficients and the number of low transfer coefficients will increase so that the ratio of the 10% quantile to the 90% quantile also increases.

In this case, a small increase in the ratio means that only a small number of transmission coefficient values below the average value are detected. If large cracks are present, more transmission coefficient values below average can be detected during the spin cycle. Thus, large fractures will result in a larger ratio of 10% quantiles to 90% quantiles than small fractures.

The optical fibers or optical fiber bundles in the inventive crack detection system can be distributed uniformly over the cross-section of the blade, but they can also be distributed non-uniformly. In particular, the amount of optical fibers or fiber bundles may be increased in areas of the blade that are more prone to cracks than others. Figure 4 shows an example where the fiber bundle density within the trailing edge 13 of the blade is increased relative to the rest of the blade.

The described crack detection system not only allows for detection of cracks in engineering structures, especially like those in wind turbine rotor blades, but also for localization and estimation of damage severity. If the system should be kept as simple as possible, it can also be designed so that crack detection can be performed with the naked eye, especially at night.

Claims (15)

1. crack detection system that is used for test load engineering structure (1) crack, this crack detection system comprises light source (17), is introduced into the optical fiber (6) in this structure (1), and the mechanism (19) that is used for the light of light source (17) is coupled into optical fiber (6)

It is characterized in that,

Optical fiber (6) has the diameter less than 75 μ m.

2. crack detection system as claimed in claim 1,

It is characterized in that,

Optical fiber (6) is introduced into this structure (1) with the form of fibre bundle (15).

3. crack detection system as claimed in claim 2,

It is characterized in that,

Fibre bundle (15) by Bao Shu permeable flexible pipe (25) or by one or more wire harness (27) of twining around fibre bundle by Bao Shu.

4. as each described crack detection system in the claim 1 to 3,

It is characterized in that,

Optical fiber (6) or fibre bundle (15) are bonded to the outside or inner of structure.

5. as each described crack detection system in the claim 1 to 3,

It is characterized in that,

Engineering structure (1) comprises the housing of being made by fiber laminate (23), and wherein optical fiber (6) or fibre bundle (15) are embedded in this laminate.

6. as each described crack detection system in the claim 1 to 5,

It is characterized in that,

Optical fiber (6) or fibre bundle (15) are connected to photodetector (21).

7. crack detection system as claimed in claim 6,

It is characterized in that,

Be connected to the modulating unit (29) that light source (17) is used to modulate its light, and

Be connected to the time gate unit (32) that detecting device (21) is used to receive detector signal.

8. as claim 6 or 7 described crack detection systems,

It is characterized in that,

Photodetector (21) is positioned at the end of optical fiber (6) or fibre bundle (15), and these ends are coupled into the terminal relative of optical fiber (6) or fibre bundle (15) with light, and

Transmission coefficient determining unit (37), the transmission coefficient of optical fiber (6) or fibre bundle (15) is determined and monitored to this transmission coefficient determining unit.

9. crack detection system as claimed in claim 8,

It is characterized in that,

Frequency detecting unit (39), it is connected to transmission coefficient determining unit (37) and is used for receiving the transmission signals of the transmission coefficient of representing optical fiber (6) or fibre bundle (15) and is designed to the frequency component of determining transmission signals.

10. crack detection system as claimed in claim 6,

It is characterized in that,

Light source (17) is the light-pulse generator of the light pulse of transponder pulse length in being lower than the 500ns scope,

These detecting devices (21) and light source (17) are positioned at the same end of optical fiber (6) or fibre bundle (15), and

Have time delay determining unit (32), it determines that light source (17) sends the time delay between pulse and detecting device (21) the reception corresponding pulses.

11. each described crack detection system of right as described above,

It is characterized in that,

Engineering structure is wind turbine rotor blade (1).

12. one kind by utilizing each described crack detection system of aforementioned claim to detect the method in the crack in the engineering structure (1), wherein monitoring is transmitted into the transmission or the reflection of the light in optical fiber (6) or the fibre bundle (15), and detects the crack by the unexpected variation of transmission or reflection.

13. method as claimed in claim 12 wherein is transmitted into light pulse in optical fiber (6) or the fibre bundle (15), and determines the time delay with respect to exomonental reflection light pulse.

14., wherein establish the time series of transmission coefficient or reflection coefficient, the Fourier analysis of the line time ordered series of numbers of going forward side by side as claim 12 or 13 described methods.

15., wherein establish the time series of transmission coefficient or reflection coefficient, and the difference of monitoring period ordered series of numbers between 10% quantile and 90% quantile as claim 12 or 13 described methods.

Images